It might be worth going back to Internet Society and asking if the
qualification on derivative works is intended to be operative or
informative. Remember you are dealing with a group that uses the term
request for comments to cover anything from reports of network outages
many years ago to the Internet protocol itself. I might be able to
make an argument that the set of things that could be a comment on an
RFC is broad enough to cover all derivative works. Certainly, if the
copyright holder intends this interpretation the license is fine.